Sunday, July 20, 2014

Species that don't get enough publicity #8. Dakosaurus, Metriorhynchus and friends

Today I’m going to talk about not a particular species, but
a family of animals. I couldn’t narrow it down to just one-collectively perhaps
only one or two has been featured in dinosaur books, and only one in my memory
has made the headlines.Remember Pristichampsus?
Well, prehistoric crocodiles are fascinating to me so you can expect more. This
time, it’s about a niche crocodilians exploited multiple times. Don’t worry, I
won’t do them all in one go. The Philodosaurs, Dyrosaurs, and Teleosaurs can
wait. Today I will restrict myself to a single but giant family of marine
crocodiles. Yes, I said marine crocodiles. These are the Metriorhynchids.

Metriorhynchus’ discovery followed Icthyosaurus only by a
few years in 1830, so I suspect it’s second-fiddle status comes from its more
prosaic origins and adapations. The three other families of marine crocodiles I
mentioned can be used as examples of transitional forms between something like
a modern crocodile and Metriorhynchus, while Icthyosaur transitional forms only
existed for a brief period of time in the early Triassic and are still unknown
in the fossil record.Metriorhynchus was
first described by Christian Von Meyer, who also described Archaeopteryx,
Rhamphorhynchus, Plateosaurus, and the previously covered Teratosaurus.

Ironically enough, the earliestMetriorhynchid found was described in 2011,
the genus Neptunidraco from Bajocian of Portomaggiore. At about the size of a
modern alligator, it was not the top predator-indeed, a giant pliosaur has been
found at the same formation. It was not the lion of the sea, but the leopard or
caracal; it ate small fish and cephalopods and was more likely to be the prey
for the pliosaurs.

You would think that the success of the pleiosaurs,
pliosaurs, and icthyosaurs would deter the crocodiles from flourishing in the
sea, but apparently they flooded the middle and late Jurassic.The Callovian stage had a species of
Metriorhynchus, two species of the short-snouted Suchodus, Maledictosuchus
from Spain, and one of
slender-snouted Gracilneustes in Europe and two of Purranisaurus in Argentina. The
Late Jurassic was the apex, however.

In the Oxford Clay, we see a faunal turnover. While Suchodus
brachyrhynchusremained
from earlier times, it was joined by two other short-snouted sea crocodiles, Aggiosaurus and Tyrannoneustes. Tyrannoneustes
is only known from a jaw, but said jaw is two feet long and armed with
broad-bladed slashing teeth. This was an animal not evolved to hunt small fish
and squid, but large prey.There’s also
a turnover in the more traditional slender-snouts as Gracilineustes acutus must give way to
Metriorhynchus superciliosus. The
plesiosaurs Cryptoclidus and Muraenosaurus, the pliosaur Liopleurodon, and Icthyosaurs like Opthalmosaurus rounded out the Oxford cast.

It’s in the Kimmeridge Clay, however, that the sea crocs reach their
apex.The 3 meter, slender snouted,
fish-trapping Cricosaurus suevicus and the similar but more generalist
Metriorhynchus geoffroyii were joined by their larger relative (5 meters) Torvoneutes,
the 6-meter apex macropredator Plesiosuchus and the smaller (4.5 meters)
macropredator Dakosaurus.

Let’s break this down. The middle-Jurassic seacrocs have slender snouts
and slender teeth. This means they were chasing numerous, fast-moving prey.
This is a typical arrangement for ichthyosaur teeth, meaning they were in
competition. What allowed them to compete is not sure, but by this time they
were fully adapted. They had lost the armor of their ancestors, narrowed their
jaws and teeth, and turned their feet into fins. The broad tail was broadened
even more and, like other marine reptiles, equipped with a fin.Icthyosaurs had rounder, shorter bodies with
dorsal fins and longer pectoral fins, while the sea-crocs had long, narrow
bodies with short fins on both sets of limbs.

Cricosaurus is a continuation of this design, and very wide-spread in
both Latin America and Western Europe. The
similar but rarer Gracilineustes made it into the Kimmeridgian as well. Equally
successful, however, is Metriorhynchus. Slightly older, it’s more successful
because the teeth are more akin to a modern crocodile-larger and stronger and
more capable of a diverse diet. While small fish and squid-like nautiloids and
ammonoids would still make up most of its diet, it would be capable of eating
large fish (perhaps even the small but very successful shark Hybodus) and
scavenging larger prey like dead plesiosaurs and Leedsicthys (a giant
filter-feeding fish that had originally shared the seas with the first sea
crocs).

There is a new group of metriorhynchids, however. These are the
Geosaurs, with shorter, broader skulls and larger, curved teeth. They ate
larger prey, like larger fish and marine reptiles.These three early genera are huge. First is
Torvoneustes; while the body, skull, and teeth are pretty similar to
Metriorhynchus, it’s almost twice the size. This suggests a more generalized
diet like Metriorhynchus, but with the size to specialize in larger prey.
Unfortunately, I’m not well-versed in fish or mollusks enough to identify
which.

Second is the reason I began this article. You see, in 1856, Theodor Plieninger.discovered teeth that belonged to
what he thought was a new species of Geosaurusafter first thinking it belonged to the dinosaur Megalosaurus. He named
it Geosaurus maximus. However, Friedrich August von Quenstedt of
the Eberhard Karls Universität Tübingen, a specialist in Jurassic fossils,
established it as Dakosaurus “Biting lizard”.

In the past 150 years, we’ve found a lot more of Dakosaurus.
While the body is similar to the other sea crocs, the skull is broader and
heavily built-the teeth have the same size as shape as a theropod dinosaur, and
as mentioned have been frequently mistaken for the teeth of Megalosaurus. These
teeth weren’t meant to pierce fish and squid; they were meant to slice flesh
with powerful bites.

However, it wasn’t a specialist predator of other marine
reptiles. That honor went to its close relative, the even bigger Plesiosuchus.
Its teeth are even more mediolaterally compressed and despite the longer snout,
the skull mechanics indicate an animal that ate large prey. Metriorhychus and
Gracilosuchus might have been prey for it! Other prey may have been the poorly-known
Plesiosaurus Kimmerosaurus, Colymbosaurus, and Bothyspondylus, the very
successful teleosaur (more on them eventually) Steneosaurus, and the
Icthyosaurs Brachypterygius and the ubiquitous Opthalmosaurus.

More on their cranical mechanics here www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0044985

However, there was one animal that still overshadowed
Plesiosuchus-the giant pliosaur Pliosaurus. Plesiosuchus’ skull may be more
than twice the size of Metriorhynchus’, but Pliosaurus’ is twice the size of
Plesiosuchus’. Perhaps it was Plesiosuchus’ rise that caused pliosaurs to grow
from the 25 foot Liopleurodon to the 40-foot Pliosaurus.Pliosaurus was the apex predator, and could
have easily taken any of the sea crocs as prey.

Finally, we have sea crocs in other places at the end of the
Jurassic, the Tithonian age. The main marine formation is the Solnhofen
Plattenkalk, a limestone formation of an archipelago in what is now
Bavaria. While the legendary pterosaurs Rhamphorhynchus and Pterodactylus and
the legendary bird Archaeopteryx flew overhead, the rich sea life continued.
Cricosaurus continued its long reign while Metriorhynchus left a descendant,
Rhaceosaurus, which had the typical slender build and relatively small size of
its ancestors and ate small fish like Gyrodus, Aspidorhynchus, Pholidophorusand Lepidotes. Geosaurus, which resembled a
more-robust headed Metriorhynchus was from a different lineage of sea crocs
more like Dakosaurus, probably took over the generalist function of
Metriorhynchus and ate larger fish like Hypsocormus.

Finally, Dakosaurus and Plesiosuchus managed to make it into
the Solnhofen as well, feeding on the larger fish and on the smaller sea
reptiles. In this case, Plesiosaurus and the ichthyosaur Aegirosaurus would
have been well within their ranges. However, as far as I can tell, no Pliosaurs
have been found from this region yet. Perhaps Pliosaurus was here as well, or
perhaps the waters proved too shallow for the giant reptile and so the
Meerckrodilers had their chance to rule.This kind of arrangement is described by Andrade and Young in their
paper High Diversity of thalatosuchian
crocodylians and niche partition in the Solnhofen sea, explaining the
respective niches of Kimmeridgian sea crocodiles.

There’s one more fauna I’d like to spotlight. In the New
World, particularly Mexico and Argentina, more marine fossils have been found.
A giant Mexican pliosaur, named the Monster of Aranberri, of unknown species was
the largest animal on the continent, but Dakosaurus was the runner up. The
Argentinan species Dakosaurus andiniensis
was nicknamed Godzilla when it was found, and was no doubt the terror of the
South seas. Dakosaurus maximus had a short, high skull, powerful jaws, and
large cutting teeth, but D. andinensis had an even boxier skull and with more
robust jaws and teeth. Perhaps the Mexican pliosaurs never came far enough
south to challenge Dakosaurus. Purranisaurus potens continued the Middle
Jurassic genus, and had many anatomical traits of the basal metriorhynchids. It
ate small squid and fish, and may have been prey itself for Dakosaurus. Finally, Cricosaurus, the very successful
metriorhynchid and possible descendant of Purranisaurus, rounded out the cast.
Plesiosaur species cannot be identified to the specific level, but Cuba has the
cryptoclidid Vinialesaurus and
the pliosaur Gallardosaurus and their
descendants may have been present in the Americans. Icthyosaurs are
represented by the large Caypullisaurus.

The Jurassic extinction took a heavy toll on marine life.
Cryptocleidids went extinct. Pliosaurs never reached the scale of Pliosaurus
again. Icthyosaurus declined to one family and fell. The Metriorhynchids joined
them. The last to survive were Geosaurus lapparenti in Valanginian France, Dakosaurus andiniensis in Berrasian Argentina, and the still-strong Cricosaurus
with its Valangian species C. saltillensis and C. macrospondylus. None made it past the
Valanginan about 135 million years ago.

What happened? One possibility is that these swift, powerful
large ichthyosaurs, the Platyptergiinae, displaced the Metriorhynchids and
ruled the oceans for another 30 million years. Another is a new group of marine
crocodiles, the Tethysuchids, replaced them. They probably evolved to fill the
niche of the extinct Teleosaurs, and had very similar anatomy. Finally, a new
shark order, the advanced Lamniformes (which were since only overshadowed by
mosasaurs and cetaceans) evolved and may have replaced both them and the
ichthyosaurs.

In pop culture they’ve played second fiddle, appearing
mostly in books than in any other media. Metriorhynchus was omitted from the
first Oxford Clay episode of Walking With Dinosaurs, but in the followup Sea
Monsters was shown joining Hybodus and Liopleurodon in devouring a Leedsicthys.
In the film Ice Age 2: the Meltdown, a Metriorhynchus and a Globidens
(separated by the same amount of geologic time than Tyrannosaurus and
humans)were the voiceless,
personality-less antagonists killed by falling boulder.In art, sea crocs have been ignored in favor
of plesiosaurs and mosasaurs; the late, great Dan Varner was a master of
mosasaurs,but I have only found one
depiction of Metriorhynchus. In terms of toys, Safari Ltd’s toobs of small
animal toys have provided two examples. In their excellent Prehistoric Sea Life
toob, each major marine group is given a representative, and Metriorhynchus
naturally represents the sea crocs. Likewise, in their also excellent
Prehistoric Crocodiles toob, the only marine archosaur among them is
Dakosaurus. I do hope large versions of
all these animals will appear in future toy lines.

I would also like to add to more papers, but some particular science
journals like to keep their ivory tower shut to poor people like me.

So anyway, Hollywood, give this crew
a break. Museums, put them on display. Toy companies, here’s a new fresh face.
Authors, think about the crap you’re writing and how this company could perk up
the place. And remember, shop Metriorhynchidae co. where you work or play.

About Me

Hi everyone! You may know me already, but 99% of you won't. I've decided to make a blog for myself. I'm a anthropology student who has returned to his original passion for palaeontology. Ever since I was little, I've been fascinated with the weird and wonderful animals that have inhabited our planet and I've made this blog to keep this in my mind and hopefully in yours. Most people blog about their interests, and while I've got a range of interests-see history and anthropology above, not to mention zoology, astronomy, art, cooking, science fiction and fantasy films and literature, and a myriad of others, the one I want to do for a living is the study of Earth's ancient past.

On this blog I'll review papers, talk about fossils, museums, and taxa, review art, film, literature, and our culture's view of paleontology, and share memories and insights. I've been inspired by the far better blogs of professional palaeontologists, and I'll share them as time goes on. I'm also open to requests and questions of opinions, the latest palaeo news, and discussions with other fans informal and professional.

I think this is going to have fun, and I'm hoping my readers will have just as much fun.

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